The invention relates to a method for detecting a d.c. fault current in an a.c. electric circuit, wherein a load with a rectifier stage having a primary side has been connected up to an a.c. network and provides a direct current on a secondary side, and wherein the a.c. network has been fuse-protected with a fault-current circuit-breaker. The invention relates, moreover, to a device for detecting a d.c. fault current in an electric circuit, to a charger including such a device, and also to the use of this charger for charging an energy storage device of a vehicle.
The electric power networks in domestic installations or for current-collection points are ordinarily protected against overload with an overcurrent protective device and include, moreover, a fault-current protective device. The overcurrent protective device interrupts the flow of current if an overload of the circuit arises, for example as a result of a short circuit. In German households the circuits are ordinarily fuse-protected in such a way that the overcurrent protective devices trip in the case of a current above 16 amperes.
The fault-current protective devices, frequently realized as fault-current circuit-breakers, interrupt the flow of current if a differential current or a fault current arises in the circuit. Such a differential current or fault current arises if the current flowing towards the load in the circuit is not equal in magnitude to the current flowing back from the load. The cause of this may be a ground fault, in the case of which a portion of the current flows back from the load via the grounding or via a protective conductor. The fault currents constitute a danger, since they point to a defective insulation which represents a danger to life and limb upon contact.
Fault-current circuit-breakers in various embodiments are typically employed by way of fault-current protective devices. In this case, fault-current circuit-breakers of type AC can detect only purely sinusoidal fault currents, and can disconnect the power supply line from the voltage source in the case of a fault.
Fault-current circuit-breakers of type A can detect both purely sinusoidal fault currents and pulsating d.c. fault currents, and can disconnect the power supply line from the voltage source in the case of a fault. Fault-current circuit-breakers of this type, which cut out in the case of a fault current greater than 30 mA, are ordinarily employed in Germany for domestic installations. If a direct current having a current intensity greater than approximately 6 mA has been superimposed on the alternating current, fault-current circuit-breakers of types AC and A no longer trip at all.
Moreover, fault-current circuit-breakers of type B are known, which can detect any type of fault current and hence can trip also in the case of a pure d.c. fault current. However, an electronic unit is required for the monitoring for such d.c. fault currents, so fault-current circuit-breakers of type B require their own power supply. Such fault-current protective devices, also called universal-current-sensitive fault-current circuit-breakers, are known in the state of the art, for example from EP 0 866 536 A2.
A disadvantageous aspect of fault-current circuit-breakers of type B is that, in comparison with fault-current circuit-breakers of type A, they are costly to produce and therefore expensive.